Blocking distortion in 18 watt amps

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Mark1406
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Blocking distortion in 18 watt amps

Post by Mark1406 »

I have looked at a few circuit diagrams where zener diodes are used to clamp the negative voltage across the 470K grid leak resistors on the output stage and I think the zener diode break over voltage is too great to kill off the crossover distortion in some drawings that I have seen on this site.

My understanding of this circuit (which might be wrong) is the zener conducts when negative voltage on the grid (with respect to earth) exceeds the Zeners break over voltage.

To put this into perspective, the signal from the P.I. stage increases till the EL-84 goes into forward conduction (the signal exceeds bias voltage), this causes the top of the signal to be clipped and for the coupling cap to charge to a negative voltage with respect to the P.I. side of the cap. Once the signal is below the cathode voltage the EL-84 ceases to conduct, though the cap remains charged as the 470K resistor is too large to dissipate the negative voltage in the required time. The grid now has a larger negative going voltage, as the signal voltage is negative going and there is the negative voltage on the cap. The combination of the negative going signal and negative DC voltage cause the valve to go into shut down prematurely when a sufficiently negative voltage is on the grid. As the other valve in the push/pull circuit is also in shut off there is a flat spot in the signal on the secondary of the output transformer.

The idea of the zener is to conduct when a certain negative voltage is reached, ideally this voltage is much less than the voltage required to send the valve into shutdown. Thus minimising the flat spot in the signal at the secondary of the output transformer.

In the circuits I have seen the zener is a 1N4744 which has a 15 volt break over voltage. This would mean that the signal would go to -15.7v with reference to earth or 28.7v with reference to the cathode of the EL-84. This seems excessive to me.

Have I misunderstood the operation of this circuit and if so could you explain the misunderstanding?

I have had the “feeling” that people have opted for the large zener diode valve as they think it will conduct before the EL-84 goes into forward conduction. However, if this hypothesis is correct, the EL-84 only conducts when the grid is more positive than the cathode (roughly 13V) and Zeners conduct when the grid is more negative than earth. Thus the Zeners can’t operate when the signal is positive going.

The other option to alleviate crossover distortion is a cathode follower. RG Keen came up with a circuit some time ago.

http://www.geofex.com/Article_Folders/m ... tfolly.htm

Thanks for your patience and I look forward to your responses.

Regards

Mark
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Re: Blocking distortion in 18 watt amps

Post by Bieworm »

Theory is one thing, but practical experience is another. The schematic from Josh indicates a zener and diode type that works. I’ve had pretty good results even with 16V zeners, but I use 13V mostly…
I wonder why this appears to be an EL84 and not a 6AQ5 issue. I’ve modded one of my Tremolo TMB amps to run 6AQ5 tubes and it doesn’t fizz at all, even without the Paul Ruby mod
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Re: Blocking distortion in 18 watt amps

Post by Mark1406 »

Bieworm wrote:
Mon 04/17/23 3:39 am
Theory is one thing, but practical experience is another. The schematic from Josh indicates a zener and diode type that works. I’ve had pretty good results even with 16V zeners, but I use 13V mostly…
I wonder why this appears to be an EL84 and not a 6AQ5 issue. I’ve modded one of my Tremolo TMB amps to run 6AQ5 tubes and it doesn’t fizz at all, even without the Paul Ruby mod
Theory and practice should be aligned. According to the JJ data sheet, when the valve is running at 250v the valve is shutdown at -15v on the grid. Running higher B+ may change matters.

As far as I can tell Paul Ruby found it occurred under specific conditions.

So using a 13v to 16v zener you could hear an improvement in crossover distortion or you could see the crossover distortion was gone by measuring the waveform with a CRO?

Regards

Mark
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Re: Blocking distortion in 18 watt amps

Post by Bieworm »

Mark1406 wrote:
Mon 04/17/23 4:05 am
Bieworm wrote:
Mon 04/17/23 3:39 am
Theory is one thing, but practical experience is another. The schematic from Josh indicates a zener and diode type that works. I’ve had pretty good results even with 16V zeners, but I use 13V mostly…
I wonder why this appears to be an EL84 and not a 6AQ5 issue. I’ve modded one of my Tremolo TMB amps to run 6AQ5 tubes and it doesn’t fizz at all, even without the Paul Ruby mod
Theory and practice should be aligned. According to the JJ data sheet, when the valve is running at 250v the valve is shutdown at -15v on the grid. Running higher B+ may change matters.

As far as I can tell Paul Ruby found it occurred under specific conditions.

So using a 13v to 16v zener you could hear an improvement in crossover distortion or you could see the crossover distortion was gone by measuring the waveform with a CRO?

Regards

Mark

340AA585-A1D5-4E5B-A4D2-FD5B8A90B6E5.png
Well, I had the crossover distortion problem, performed the ruby mod using various zener voltage values and could get up to a value of 18V without the problem popping up. No further investigation needed.. my ears told me things were fine.
The science behind this is beyond my knowledge and I don't worry about it. :D
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Re: Blocking distortion in 18 watt amps

Post by Mark1406 »

Bieworm wrote:
Mon 04/17/23 5:44 am
Mark1406 wrote:
Mon 04/17/23 4:05 am
Bieworm wrote:
Mon 04/17/23 3:39 am
Theory is one thing, but practical experience is another. The schematic from Josh indicates a zener and diode type that works. I’ve had pretty good results even with 16V zeners, but I use 13V mostly…
I wonder why this appears to be an EL84 and not a 6AQ5 issue. I’ve modded one of my Tremolo TMB amps to run 6AQ5 tubes and it doesn’t fizz at all, even without the Paul Ruby mod
Theory and practice should be aligned. According to the JJ data sheet, when the valve is running at 250v the valve is shutdown at -15v on the grid. Running higher B+ may change matters.

As far as I can tell Paul Ruby found it occurred under specific conditions.

So using a 13v to 16v zener you could hear an improvement in crossover distortion or you could see the crossover distortion was gone by measuring the waveform with a CRO?

Regards

Mark

340AA585-A1D5-4E5B-A4D2-FD5B8A90B6E5.png
Well, I had the crossover distortion problem, performed the ruby mod using various zener voltage values and could get up to a value of 18V without the problem popping up. No further investigation needed.. my ears told me things were fine.
The science behind this is beyond my knowledge and I don't worry about it. :D
Well if it worked for you there are no complaints. :D

Me being me, I just like to know how something is supposed to work before I use it. I want to have some idea about the results that I expect to get.

Regards

Mark
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Re: Blocking distortion in 18 watt amps

Post by JMPGuitars »

Mark1406 wrote:
Mon 04/17/23 7:51 am
Well if it worked for you there are no complaints. :D

Me being me, I just like to know how something is supposed to work before I use it. I want to have some idea about the results that I expect to get.

Regards

Mark
Don't worry, theory and practice do get along. Read the doc here: storage/18-watter_buzz_info_311.pdf

In the doc you'll see Paul's cathode voltage was 7V, so he went a couple volts higher at 9.1 for the diodes he chose. Our cathode voltage is typically around 12V (targeting 85% dissipation at the power tubes), so we go a few volts higher at 15V. His troubled amp was a conversion amp, not a typical 18W.

As you mentioned, raising the voltages on the tubes changes the behavior. What you expect at 250V plates is not what you expect at 350V plates. If you have a tube curve tracer, you can see how every sweep is different. Without that, you're speculating without supporting data.

The noise this issue creates is fairly distinctive. If you hear it, chances are, the PR mod will remove the unwanted noise. It's not a subjective or superficial practice.

For some DSO scoping check this video out: https://youtu.be/URrVYnpaE5c - this guy is f'n crazy, don't do anything he does lol. Also, I didn't watch the whole video, I don't know everything he says. I do know that he shows examples of the crossover distortion on the DSO. Sorry it isn't a CRO. 😉

Thanks,
Josh
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Mark1406
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Re: Blocking distortion in 18 watt amps

Post by Mark1406 »

JMPGuitars wrote:
Mon 04/17/23 8:37 am
Mark1406 wrote:
Mon 04/17/23 7:51 am
Well if it worked for you there are no complaints. :D

Me being me, I just like to know how something is supposed to work before I use it. I want to have some idea about the results that I expect to get.

Regards

Mark
Don't worry, theory and practice do get along. Read the doc here: storage/18-watter_buzz_info_311.pdf

In the doc you'll see Paul's cathode voltage was 7V, so he went a couple volts higher at 9.1 for the diodes he chose. Our cathode voltage is typically around 12V (targeting 85% dissipation at the power tubes), so we go a few volts higher at 15V. His troubled amp was a conversion amp, not a typical 18W.

As you mentioned, raising the voltages on the tubes changes the behavior. What you expect at 250V plates is not what you expect at 350V plates. If you have a tube curve tracer, you can see how every sweep is different. Without that, you're speculating without supporting data.

The noise this issue creates is fairly distinctive. If you hear it, chances are, the PR mod will remove the unwanted noise. It's not a subjective or superficial practice.

For some DSO scoping check this video out: https://youtu.be/URrVYnpaE5c - this guy is f'n crazy, don't do anything he does lol. Also, I didn't watch the whole video, I don't know everything he says. I do know that he shows examples of the crossover distortion on the DSO. Sorry it isn't a CRO. 😉

Thanks,
Josh
Thanks for taking time to reply to my question Josh.

I don’t own a valve curve tracer, I imagine they are a pretty rare piece of equipment these days.

My question is why do you want the zener to be a few volts more than the cathode voltage?

In a perfect world the push/pull stage would cross over at 0 volts. However the coupling cap charging is forcing the grid to go negative with respect to ground. Paul Ruby said his crossover point was -17vgk, which is -10v with reference to earth.
The zener diode and reverse diode limit the negative excursion of the signal.
How is the voltage on the cathode significant?

Regards

Mark
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Re: Blocking distortion in 18 watt amps

Post by JMPGuitars »

Mark1406 wrote:
Mon 04/17/23 9:09 am
Thanks for taking time to reply to my question Josh.

I don’t own a valve curve tracer, I imagine they are a pretty rare piece of equipment these days.

My question is why do you want the zener to be a few volts more than the cathode voltage?

In a perfect world the push/pull stage would cross over at 0 volts. However the coupling cap charging is forcing the grid to go negative with respect to ground. Paul Ruby said his crossover point was -17vgk, which is -10v with reference to earth.
The zener diode and reverse diode limit the negative excursion of the signal.
How is the voltage on the cathode significant?

Regards

Mark
Take a look through the PDF again, he discusses that.

Check this thread for a modern curve tracer: viewtopic.php?f=12&t=25932

Thanks,
Josh
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